The present application relates to a beam applying method and a beam applying apparatus that records or reproduces a signal in or from an optical recording medium in or from which a signal is recorded or reproduced by applying a beam thereto, and an optical recording medium.
In a hologram recording and reproducing system in the field of optical storage, a spatial light modulator (SLM) such as a transmissive liquid crystal panel and a digital micro mirror device (DMD) is used as a light intensity modulator and the intensity of a signal beam is modulated to obtain a pattern arrangement of bit1 (for example, light intensity=high) and bit0 (for example, light intensity=low).
At this time, the SLM generates the signal beam by modulating the light intensity of a beam at the center thereof on the basis of recording data and generates a reference beam by allowing a beam to pass through the periphery thereof in a ring shape. The signal beam modulated on the basis of the recording data is applied to a hologram recording medium along with the reference beam, whereby an interference pattern of the signal beam and the reference beam is recorded as data in the hologram recording medium.
At the time of reproducing data, a diffracted beam corresponding to the interference pattern is obtained by allowing the SLM to generate only the reference beam and applying the generated reference beam to the hologram recording medium. An image corresponding to the diffracted beam is formed on an image sensor such as a CCD (Charge Coupled Device) sensor and a CMOS (Complementary Metal-Oxide Semiconductor) sensor and values of the recorded bits are obtained, thereby reproducing the data.
Accordingly, the hologram recording and reproducing system in which a signal beam and a reference beam are applied in the same optical axis is known as a coaxial system.
Here, in the hologram recording and reproducing system, when a reflecting hologram recording medium (a hologram recording medium having a reflecting film) is used as the hologram recording medium, optical technologies of an optical disk such as a CD (Compact Disc) or a DVD (Digital Versatile Disc) can be sufficiently used, but a hologram formed by applying a signal beam and a reference beam is complicated.
FIGS. 20A, 20B, 21A, and 21B show patterns of holograms which can be formed at the time of recording data in a reflecting hologram recording medium.
As shown in FIGS. 20A to 21B, total 4 patterns of holograms can be formed when data are recorded in the reflecting hologram recording medium:
Pattern A; signal beam (forward path)×reference beam (forward path) =transmissive hologram
Pattern B; signal beam (forward path)×reference beam (backward path)=reflective hologram
Pattern C; signal beam (backward path)×reference beam (forward path)=reflective hologram
Pattern D; signal beam (backward path)×reference beam (backward path)=transmissive hologram
Specifically, the transmissive hologram of pattern A shown in FIG. 20A is a hologram formed by means of interference of both forward beams of a signal beam and a reference beam applied to the hologram recording medium through an objective lens as shown in the figure. The reflective hologram of pattern B shown in FIG. 20B is a hologram formed by means of interference of a forward beam of a signal beam applied to the hologram recording medium through the objective lens and a backward beam of a reference beam reflected from the hologram recording medium.
The reflective hologram of pattern C shown in FIG. 21A is a pattern opposite to pattern B, that is, a hologram formed by means of interference of a forward beam of the reference beam applied to the hologram recording medium through the objective lens and a backward beam of the signal beam reflected from the hologram recording medium. The transmissive hologram of pattern D shown in FIG. 21B is a hologram formed by means of interference of both backward beams of the reference beam and the signal beam reflected by the hologram recording medium.
The 4 holograms are difference from each other in characteristics of the interference pattern due to differences in traveling direction and angle and have different selectivity for medium shift and wavelength shift (for example, see M. Toishi et al. Appl. Opt., Vol. 45, No. 25, p. 6367 (2006)). Accordingly, it is not easy to correction the characteristics in the shifts, thereby causing a deterioration in SNR (S/N ratio).
A technique disclosed in U.S. Patent Application Publication No. 2003/0039001 is known to solve the above-mentioned problem.
In U.S. Patent Application Publication No. 2003/0039001, as shown in FIG. 22A, a quarter-wavelength plate is inserted as a layer in front of a reflecting film in the reflecting hologram recording medium. That is, a cover glass, a recording layer, and a reflecting film are formed sequentially from the uppermost layer in a general reflecting hologram recording medium, but the quarter-wavelength plate is inserted between the recording layer and the reflecting film in this case.
By using the above-mentioned hologram recording medium, it is possible to effectively prevent the reflective hologram from occurring. This is shown in FIG. 22B. As shown in the figure, for example, an X-linear polarized beam is applied to the hologram recording medium through the objective lens. The X-linear polarized beam applied to the hologram recording medium passes through the recording layer of the hologram recording medium, is converted into a right-rotated circularly-polarized beam as shown in the figure by passing through the quarter-wavelength plate, and then reaches the reflecting film below the quarter-wavelength plate. The circularly-polarized beam reaching the reflecting film is reflected therefrom and passes through the quarter-wavelength film again. Accordingly, the reflected beam from the hologram recording medium is obtained from the Y-linear polarized beam as shown in the figure.
According to the technique described in U.S. Patent Application Publication No. 2003/0039001, the forward beam to the hologram recording medium can be obtained by the use of the X-linear polarized beam and the backward beams as the reflected beam can be obtained by the use of the Y-linear polarized beam. That is, polarization directions of the forward beam and the backward beam are perpendicular to each other to prevent the interference of the forward beam and the backward beam, thereby effectively preventing the reflective hologram resulting from pattern B and pattern C.